Microfilm copying is a process wherein photographically reduced images of documents are sequentially formed, usually in successive frames on a reel of film, which is thereafter removed from the copy area. During this process, the film transport speed required for normal microfilming is directly proportional to the optical reduction ratio. In a rotary microfilmer, the document and film are in continuous motion during the microphotographing process.
Depending on the type of material being copied, different magnifications are desired, which is typically accomplished by manually replacing lenses of different focal lengths. There are a number of what can be considered standard reduction ratios in the industry, namely 24X, 40X and 50X; with some reduction ratios going as high as 150X. The selection of a particular ratio is based on a trade-off between clarity and ease of viewing the magnified image for the lower ratios; while the higher packing density of images on the film favors the higher reduction ratios.
In a so-called rotary microfilmer machine, the document and film are in continuous motion during the microphotographing process and the exposure is made through a narrow slit in a plate located in the optical path between the document and the film. Consequently, the size of the image in the direction of relative movement between the document and the paper is dependent on the relative speed of movement between those elements, which must be adjusted to maintain the proper aspect ratio of the copy when the magnification is changed.
The micrographics industry has relied on two techniques in the past to change from one optical reduction ratio to another. One technique involves replacing the entire camera subassembly comprising the lens and mechanical drive components. Thus, each camera subassembly is dedicated to one and only one reduction ratio. Such an arrangement is bulky, requiring additional storage space as well as being costly, requiring the duplication of a number of parts in each camera subsassembly. The second technique uses two or three lenses adjacently mounted on a carriage controlled by a lever that will move the appropriate lens on the carriage into position. It is also common that the same lever is used to select the film transport speed ratio by shifting gears or changing pulleys to adjust the transport and film speeds so that they correspond to the reduction ratio selected.